High purity or doped fused silica glass materials have found use in many modern optical systems. For example, state-of-the-art lithography devices in the semiconductor industry, especially those operating at deep UV and vacuum UV wavelengths, typically have refractive or reflective optical elements made from or based on high purity or doped fused silica glass materials. The high precision and resolution required for modern lithography devices in the production of very large scale integrated circuits has pushed the lithographic radiation to shorter wavelength, thus higher energy. The optical elements used in these systems are required to have high performance in many aspects, such as transmission, transmission degradation, birefringence, refractive index homogeneity, laser-induced wavefront distortion (LIWFD), thermal expansion coefficient uniformity, and the like.
Doped and undoped Synthetic silica glass materials have also been used in the manufacture of optical fibers.
Two types of processes for making synthetic silica materials have been developed and commercialized to date. They are the direct-to-glass process and soot-to-glass process. In the former, glass soot particles are provided by, e.g., flame hydrolysis of a silicon-containing precursor material and optional dopants, deposited on a supporting surface and sintered in situ into transparent glass. In a soot-to-glass process, a glass soot preform is first formed from the glass soot particles, then optionally dried, purified and/or doped, before it is sintered into dense glass in a separate step. Various soot deposition techniques have been developed, such as outside vapor deposition (OVD), inside vapor deposition (IVD) and vapor axial deposition (VAD), and the like. All these processes were initially designed for making waveguide preforms. Because of the deposition techniques used, undesired property inhomogeneity in the sintered glass tend to exist, and post-sintering reforming of the glass are invariably required in order to produce the glass meeting the requirements of modern UV lithography.
U.S. Pat. No. 6,606,883 discloses soot-to-glass process for making doped and un-doped silica glass in which soot particles are deposited on a planar surface. However, the teaching in this reference of the process and apparatus is limited, especially with regard to the soot density variation of the soot preform produced therein. Moreover, the examples in this patent reference involve the use of refractory bricks in constructing the furnace, which are potential contaminant sources.
The present invention is an improvement over the process and apparatus described in U.S. Pat. No. 6,606,883.